This invention relates to sonar transducers; and more particularly to a conical beam shock-hardened, high pressure, ceramic sonar transducer for use in hostile environments.
Sonar is a term that generally refers to a system that uses underwater sound, at sonic or ultra-sonic frequencies, to detect and locate objects in the sea, or for communication. A sonar transducer is a device used underwater to convert electrical energy to sound energy, as when underwater sound is being generated and transmitted by the transducer, or for converting sound energy to electrical energy, as when the transducer is being used to intercept and amplify underwater sound signals.
At the heart of every sonar transducer is some form of piezoelectric element that undergoes a dimensional change when stressed electrically by an external voltage or that generates an electrical change when stressed mechanically by an external force. A popular, and relatively common, type of piezoelectric element is the piezoelectric ceramic. The piezoelectric ceramic may take a wide variety of forms, ranging from cylinders, discs, bars, or spheres. These ceramics may be configured to be sensitive to mechanical stress, or to undergo dimensional change only in a selected axis. For example, a conical beam sonar transducer--that is, a transducer designed to transmit and receive underwater sound energy only in one direction--will advantageously have the sensitive axis of the piezoelectic ceramic positioned so as to be aligned with the desired directivity of the transducer. Amplification may be achieved by connecting several such transducers in parallel, each having its sensitive axis in parallel with the desired direction of directivity. When the transducer is to generate, or transmit, a signal in the desired direction, the piezoelectric elements are jointly stressed by an appropriate voltage. This voltage causes each element to undergo a dimensional change along its sensitive axis, which dimensional change is in turn, coupled through a suitable transfer medium to the water immediately in front of the transducer. The dimensional change therefore causes the water to alternately be subjected to compression and tension forces. Such forces cause a positive or negative pressure wave (sound) to travel through the water, originating at the sonar transducer and traveling out therefrom according to well known principles of wave propogation theory.
In a similar fashion, albeit reversed, when a sound wave is traveling through the water in the direction of the sensitive axis of the transducer, and the sound wave strikes the transducer, a positive or negative pressure is coupled through the transducer to the piezoelectric ceramics. This pressure causes the ceramics to undergo a mechanical stress, which stress generates an electrical charge that can be sensed through appropriate electrical means.
Unfortunately, piezoelectric ceramic elements tend to be very fragile, and are easily broken or shattered when subjected to extreme pressures or explosive or severe mechanical shocks. Such pressures and shocks may be readily encountered by a sonar transducer mounted on a submarine, or other submersible, employed in war-time service. Prior attempts to protect the ceramic elements from such high pressures and explosive mechanical shocks have resulted in serious inefficiencies in the operation of the transducer. For example, prior art ruggedizing techniques used in connection with conical beam transducers have either reduced the sensitivity and/or adversely affected the directivity pattern associated with the transducers.
A further problem associated with piezoelectric ceramic elements of the type commonly used in sonar transducers is their susceptibility to damage when operated in shallow depths at higher than cavitation levels. Cavitation, as used herein, refers to the tendency of the water to literally break apart when subjected to a tension force of sufficient strength. At shallow levels, where the hydrostatic pressure levels are relatively low, the tension force sufficient to pull the water apart may be generated by the sonar transducer. If this occurs, there is a tremendous mismatch between the sonar transducer and the cavitated water, resulting in a tremendous amount of energy that remains trapped inside of the transducer. This energy may cause serious damage to occur to the ceramic element.
A further problem of any new replacement transducer adapted for operation in hostile environments is that it be interchangeable and compatible with existing sonar transducers. This is because sonar transducers, of the type discussed herein, are typically used with expensive submarines or other large and complex submersibles that have long been designed to be used only with a transducer having a prescribed configuration, both mechanically and electrically. Thus, any ruggedized replacement transducer must be compatible with existing mounting structure, as well as existing and control circuitry, if the transducer is to be a viable replacement.